Many catalyst systems and production processes are known for the production of high cis (that is, greater than about 90%, more preferably greater than 95% in the cis formation) polybutadienes. These processes typically involve the polymerization of 1,3-butadiene in an inert liquid polymerization medium in the presence of a homogeneous catalyst system. The catalyst system typically comprises a transition metal (principally cobalt) salt with an alkyl aluminum halide.
For example, U.S. Pat. No. 3,135,725 teaches a high cis polybutadiene can be produced by polymerizing 1,3-polybutadiene in an inert solvent in the presence of a catalyst which contains cobalt in complex combination with an alkyl aluminum chloride.
Recently, European Patent 0,652,239 B1 disclosed a process for producing high-cis polybutadiene comprising polymerizing 1,3-butadiene in an inert hydrocarbon solvent together with water and a catalyst system comprising (in specified ratios) (1) a substantially anhydrous divalent cobalt salt CoAm, where A is a monovalent or divalent anion of the salt and m is 0 or 1; (2) diethyl aluminum chloride or ethyl aluminum sesquichloride and (3) an organo aluminum compound of the formula R3Al, wherein R is an alkyl group having from 8-12 carbon atoms (and optionally triethyl aluminum). The addition of the trialkyl aluminum compounds was said to reduce the level of gel formation in the reaction product.
Currently, cobalt dioctoate is the most commonly used source of cobalt in the industry. This is also the preferred Cobalt salt in EP 0,652,239. Similarly, the most commonly used, and EP 0,652,239's preferred organo aluminum chloride species is diethyl aluminum chloride (“DEAC”). This is probably due in part to cobalt dioctoate's relatively high solubility in DEAC. It has been observed, however, that DEAC promotes branching in the polybutadiene which leads to the formation of gels, causing fouling on the reactor surfaces. Increased fouling requires the reactors to be shut down for maintenance more frequently.
Accordingly, it is an objective of the present invention to provide a catalyst system which reduces branching and fouling without a substantial decrease in the catalyst activity.
It has been discovered that the use of ethyl aluminum sesquichloride together with trioctyl aluminum produces a more linear product and exhibits less fouling that when DEAC alone is used. This effect is somewhat offset by slower conversion rates which were observed. These conversion rates were improved however, by using cobalt neodecanoate as the cobalt salt. Furthermore it was observed that the activity of all of these cobalt systems could be improved by the addition of an amount of ternary alkyl or aryl amines. Thus it was possible to achieve similar conversion rates, while simultaneously reducing branching and reducing fouling of the reactor.
Accordingly, one aspect of the invention is the use of ethylaluminum sesquichloride and trioctylaluminum as co-catalyst with a cobalt salt. Another aspect of the present invention is the use of cobalt neodecanoate as the cobalt salt. Yet another aspect of the present invention relates to the use of ternary alkyl amines or ternary aryl amines as an additive to a catalyst system which comprises a cobalt salt together with an organo aluminum halide.
Other references worth considering including U.S. Pat. No. 5,905,125 which discloses a catalyst system comprising a cobalt salt, trialkyl aluminium, aluminium halide and water; U.S. Pat. No. 5,733,835 which discloses a catalyst system comprising an organo- cobalt compound, trialkyl aluminium and hexafluoro-2-propanol, and U.S. Pat. No. 3,336,280 which discloses a catalyst system comprising molybdenum pentachloride, organo-aluminium and the use of amines as “promoters”.